Green tides of macroalgae have been negatively affecting the coasts of Brittany, France, for at least five decades, caused by excessive nitrogen inputs from the farming sector. Regular areal estimates of green tide surfaces are publicly available but only from 2002 onwards. Using free and openly accessible Landsat satellite imagery archives over 35 years (1984–2019), this study explores the potential of remote sensing for detection and long-term monitoring of green macroalgae blooms. By using a Google Earth Engine (GEE) script, we were able to detect and quantify green tide surfaces using the Normalized Difference Vegetation Index (NDVI) and Normalized Difference Water Index (NDWI) at four highly affected beaches in Northern Brittany. Mean green tide coverage was derived and analyzed from 1984 to 2019, at both monthly and annual scales. Our results show important interannual and seasonal fluctuations in estimated macroalgae cover. In terms of trends over time, green tide events did not show a decrease in extent at three out of four studied sites. The observed decrease in nitrogen concentrations for the rivers draining the study sites has not resulted in a reduction of green tide extents.
- Coastal waters
- Earth observation
- Google Earth Engine
- Remote sensing
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Schreyers, L. (Creator), van Emmerik, T. (Creator), Le Lay, Y. (Creator) & Biermann, L. (Creator), Wageningen University & Research, 1 Mar 2021
Template scripts on Google Earth Engine to detect green tides on the foreshores of beaches using Landsat archive
Schreyers, L. (Creator), Wageningen University & Research, 17 Feb 2021